test: note that the square bracket form is available

As requested in https://github.com/fish-shell/fish-shell/issues/1850
Update pythonista.fish: make virtualenv aware that prompt is set

Closes #1823, closes #1795.

Signed-off-by: David Adam <zanchey@ucc.gu.uwa.edu.au>
Change fish_paginate to paginate both stdout and stderr.
Miscellaneous cleanups related to multithreaded execution
Migrate exit status into env_stack_t to make $status work
Rename events and simplify signal event handling

- Rename 'events' to 's_event_handlers'
- Stop inspecting the s_event_handlers list upon receiving
a signal. Instead, maintain the set of signals that are observed
in a separate static array. This lets us avoid mucking with
STL data structures in a signal handler, and so avoid blocking signals
in event.cpp
Merge branch 'master' into multithreaded_execution